Wax nostalgic about and learn from the history of early electronics.
See articles from Radio-Craft,
published 1929 - 1953. All copyrights are hereby acknowledged.

This Radio Service Data Sheet for the Sparton Model 40 6-Tube
T.R.F. Automotive Receiver is an example of the dozens of similar
schematic and alignment instruction sheets that have been posted
on RF Cafe over the years. Obtaining technical information on
most things, even readily available items, prior to the Internet
era was often very difficult - if not impossible. Service centers
had what was need provided by manufacturers and distributors,
but if you wanted to find a part number or service data on a
refrigerator, radio, lawn mower, garage door opener, etc., and
did not have the original paperwork, you were usually out of
luck. Nowadays a Web search will quite often get you what you
need thanks to people (like me)
who go to the trouble of making the information available. The
stuff doesn't just magically appear or get posted by benevolent
governmental entities. You're welcome.

To maintain constant signal output, regardless of the intensity
of the incoming signal (within practical limits), to overcome
the reduction in signal intensity which will occur in a given
locality (due to metallic structures, ore deposits, etc.) it
is necessary to incorporate some form of volume control which
will operate to vary the gain in the amplification of the receiver
in proportion to the loss in carrier signal strength. Most automatic
volume controls or A.V.C. circuits operate to vary the control-grid
of the amplifier tubes, in accordance with the A.F. modulation
of the station's carrier; the "Lafoy" system, however, varies
the control-grid bias more nearly in accord with the intensity
variations of the station's carrier itself, the A.V.C., tube
V6 in the diagram, functioning more nearly as an R.F. amplifier
than as a detector.

High amplification in this set is obtained through the use
of a three-stage R.F. amplifier incorporating screen-grid tubes
of the "automotive" type, the output of this section feeding
a screen-grid detector. The audio circuit comprises a single
pentode, which is impedance- and resistance-capacity-coupled
to the detector.

Correct methods for installing and servicing antennas and
interference suppressors have been described in past issues
of Radio-Craft. However, a little additional data is available.

For the aerial in collapsible type tops, we recommend that
the "false top" type be employed. This type of aerial is constructed
in the following manner:

Fashion two pieces of drill cloth that are the same color
as the top material, as long as, and approximately six inches
narrower than the roof. On one section, lay a piece of light
weight felt of the same dimensions, and then lay on top of the
felt a piece of 16-mesh copper screen wire the same size. On
top of this wire, lay another piece of light weight felt and
over this the second section of drill cloth, then sew the edges
of the combination together.

The top deck is removed from the roof bows and the aerial
is placed on top of thorn. The top decking is then placed back
over the aerial.

where it is desired to let the top down, it is advisable
to connect the aerial lead-in wire to the aerial at the rear,
and let the shielding on this wire, extend only for a distance
of about three feet from the receiver end. In such cases, the
lead-in wire is run through the floor boards back of the seat
underneath the car, up to the receiver.

Note that in this receiver there are two fuses; one of them
is of 1/8-A. rating, and is connected in the "B" battery jumper
wire, while the other is a 5 A unit located in the receiving
unit near tile ground binding post.

Interference may be distinguished by the sound: Generator
noises (eliminated by bypassing the commutator) are tone frequencies
quite different from the staccato tapping sound of high-tension
spark interference ; high-tension interference is a sharp, raspy
sound and can be eliminated practically 100 percent by means
of spark suppressors (on the distributor and spark plugs); low-tension
breaker point noise is not readily distinguished from high-tension
interference, but will be the sound remaining after spark suppressors
have been installed. Low-tension breaker interference is difficult
to eliminate. Try reversing the two primary leads to the coil;
install a bypass condenser connected from the engine to the
ammeter and switch lead.

Sketch of the battery box showing the location of the cable
and the battery.

The operating voltage and current characteristics of this
set are to be measured with a set analyzer equipped with a voltmeter
of the 1,000-ohms-per-volt type; the manual volume control must
be turned to the full on position, and with no signal reception.

The antenna compensating condenser C5 is to be adjusted at
the time the receiver is installed. Tune in a weak station between
1200 and 1400 kc., turn the volume control full on, and then,
using an insulated screwdriver, adjust C5 for maximum receiver
output. Never adjust either the C5, or the remaining trimmers,
with the cover removed.

Circuit oscillation can be caused either by tubes or the receiver
itself. Check the contact surfaces between the partitions and
the rotor shaft, making sure that a good ground is obtained.
Do not under any conditions oil the shaft under the contacts.
Make sure that the receiver chassis is well grounded.

For best results it is essential that the receiver unit be
located so that the remote control flexible-shaft runs direct
(that is, without sharp bends).

The battery circuit for this receiver is unusual, as indicated
in the diagram; an additional figure illustrates the connections.

The "A" battery consumption of this set is about 2 1/2 A.;
the "B" requirements, about 20 ma.

Complete schematic circuit of the Spartan model 40 receiver
using the new Lafoy system of automatic volume control.

Posted September 15, 2015

Radio Service Data Sheets

These schematics, tuning instructions, and other data are reproduced from my
collection of vintage radio and electronics magazines. As back in the era, similar
schematic and service info was available for purchase from sources such as
SAMS Photofacts, but these printings
were a no-cost bonus for readers. There are 220 Radio Service Data Sheets as of
February 2, 2018.

RF Cafe began life in 1996 as "RF Tools" in an AOL screen name web space totaling
2 MB. Its primary purpose was to provide me with ready access to commonly needed formulas
and reference material while performing my work as an RF system and circuit design engineer.
The Internet was still largely an unknown entity at the time and not much was available
in the form of WYSIWYG
...

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used on the RF Cafe website are hereby acknowledged.